Support Histograms

[abakirov]

Histograms are almost like bar charts except X-axis will show numeric intervals with some predefined breaks (equal intervals, natural breaks, etc.).

The implementation could probably use getHistogram method from new JS API.

[abakirov]

It would be cool if histograms were also interactive allowing users to select subsets of data using double slider. One of the many examples of how others do this: https://dev.anuary.com/703dff31-b9be-543f-b6c6-61ae7fb4dd32/

(although this should probably fall into bigger topic of how to handle selections in different types of charts)

Please share thoughts on this.

[ajturner]

Vega 2 provides interaction and selection. I'm talking with the team about upgrading

[ajturner]

good progress - but a few things to polish up in #122